1. Field of Art
The present invention relates to rolling method and apparatus, and more particularly to a rolling technology which assures a symmetrical cross sectional shape of each of successive projections which are defined by crests and roots formed on a rolled blank, and which particularly assures a cross sectional symmetry of each crest of such projections or teeth generated on a cylindrical blank.
2. Description of the Prior Art
In the art of generating various forms such as projections, teeth, splines and serrations on the periphery of a blank, a rolling process is known, wherein the blank is rolled on the periphery of shaped rolling dies while the rolling faces of the dies are held in pressed contact with the peripheral surface of the blank, whereby projections or teeth consisting of crests and roots corresponding to a desired form to be obtained on an end product is formed on the periphery of the blank. In such a rolling operation, the crests and roots are formed such that the corresponding teeth of the dies penetrate the blank and displace the material to form the roots of the projections to be obtained, and then the displaced material between the roots is forced radially outward to form the crests of the projections. While each of the crests is formed, the material is displaced under pressed sliding actions between the faces of the crest and the mating faces of the die tooth, and flows along those faces in the rolling direction in order to form the crest into a desired final shape (tooth form) in cross section across the axis of the blank.
It is recognized that the direction and amount of flow of the material are determined by the direction of sliding action of the die tooth on the faces of the crest and by the surface pressure between the mating faces of the die tooth and the crest. In the conventional rolling method, however, a blank is rolled in one direction only, i.e., rolled unidirectionally, and therefore there arise differences in the sliding direction and surface pressure between the right and left faces of the die tooth and of the crest being formed on the periphery of the blank. This unevenness in sliding direction and surface pressure causes a problem that each of the crests formed on the blank isnot symmetrical in cross section (cross sectional profile) taken in a plane across the axis of the blank. In general, most of the blank material displaced by the teeth of rolling dies tend to flow radially outwardly along the faces of a crest being formed on the blank, due to sliding contact under pressure between the faces of each die tooth and the corresponding faces of the crest, thus causing shoulder portions or edges of a top land of the crest to radially outwardly protrude from the top land. According to the conventional unidirectional rolling method, the formation of such a radial protrusion occurs in greater amount at one of the shoulder portions adjacent to one edge of the top land, than at the other shoulder portion, and this greater protrusion in the form of burrs must be removed in a subsequent finishing process during or after a rolling cycle, which finishing process is troublesome. Alternatively, an attempt has been made to press such burrs onto the top land of the crest to re-form the crest. In this instance, a product having rolled peripheral teeth with such re-formed crests is unsatisfactry in accuracy and strength.
The above indicated inconveniences experienced in the prior art rolling will be aggravated as the amount of displacement of the blank material is increased, that is, as the number of teeth (projections) to be formed on the blank is reduced or as the depth of the teeth is increased.
As a result of various researches and studies in view of the above background, inventors of the present invention have rejected a common recognition in the prior art that rolling dies located opposite to each other with a given distance therebetween are not allowed to stop their relative movements (rolling action of a cylindrical blank) during a rolling cycle, and that the rolling cycle must be completed during one relative movement of one die to the other in one direction. More particularly, the inventors have found a fact that a cross sectional profile of each tooth form to be generated on the periphery of the blank is improved by reciprocating the rolling dies relative to each other in opposite directions and concurrently feeding the blank axially to push the same from its one end between the rolling dies. The present invention was developed based on the above recognition and knowledge.